Drilling mud



. the hole.

United States Patent DRILLING MUD Paul W. Fischer, Whittier, and RaymondA. Rogers, Fullerton, Calif., assignors to Union Oil Company ofCalifornia, Los Angeles, Calif., a corporation of California No Drawing.Application January 19, 1953 Serial No. 332,102

7 Claims. (cl. 252-85) This invention relates to the treatment ofdrilling fluids such as are employed in the drilling of oil and gaswells.

In general a drilling fluid, particularly that employed for rotarydrilling operations, which is also termed a drilling mud or simply amud, is a colloidal suspension of a suspending agent such as bentoniteor other suitable clays in water to which other materials such as bariumsulfate may be added in order to increase its apparent specific gravity.The physical charactertistics of these clay suspensions are largelydetermined by the properties of the individual clay particles, mostimportant of which are size, shape and surface characteristics.

Until recently it has'been considered that the principal.

functions of a mud in drilling operations are three-fold:

2,8573% Patented Oct. 21, 1958 hole may cause dangerous caving.Therefore, mud is usually pumped into the well while removing the drillstring in order to maintain a substantially constant hydrostatic head onthe formations being drilled and the mud gravity is'always kept asconstant as practical. In

7 general, the practice has been to keep the weight of mud onlysulficiently high to prevent blow-outs. It is common practice toincrease the specific gravity of a mud by adding finely groundinsolublematerials of high density, such as for example, barium sulfate, ironoxide, etc. Because of their insolubility, theusual weighting agentsemployed have little effect on the performance characteristics of a mud.

Of the various characteristics of a drilling mud, the most important isits tendency to lose water to the formation. It has been found that thestructural strengths of most formations which are penetrated duringdrilling are sufiicient to prevent the walls of the hole from caving,but that many such formations are weakened from being saturated or evenpartially saturated with water. Frequently such action causes sloughingof the formation or tools so that they cannot be removed and costlyfishing it should form a cake on the wall of the hole; it should retainin suspension the cuttings formed so that the latter may be removed fromthe hole along with the mud; and it'should possess sufiicient weight toovercome any pressure encountered during drilling. These functions areconsidered in more detail below.

. One of the primarydifliculties encountered in the drilling of Wells isthat due to the sloughing or caving into the hole of the formationspenetrated. The use of a to allow it to be readily pumped and it shouldbe thixotropic. Thixotropy is that property of colloidal suspensionwhich involves an increase in gel strength as a function of the time ofquiescent standing. This property is valuable in that it prevents, to alarge extent, the sedimentation of the cuttings in the hole duringperiods of suspended circulation. However, immediately after Violentagitation, such as is induced by the circulation of the mud, cuttingswill settle a short distance and this fact is utilized for their removalin a settling tank pro- -vided for the purpose and in which the degreeof agitation is suddenly lowered to practically zero. Normally, a mudwhich has proper wall building characteristics and is pumpable will haveadequate thixotropic properties.

Obviously, the total weight of a mud must be sufliciently great toprevent blow-outs from any high pressure formations that may beencountered, but beyond this point, the need for greater weght isproblematical. A rapid reduction in the hydrost:.tic head maintained onformations, caused by a reduction in the specific weight of the mud orby permitting the level of the mud in the well to fall while removingthe drilling tools from the The mud must be of sufficiently lowviscosity drilling mud is supposed to lessen the tendency for cavjobsresult. It is, therefore, of the greatest importance to prevent loss ofwater from the drilling mudto the formations drilled. The ideal drillingfluid should permit very little if any loss of water to the formationand should deposit only a relatively thin mud cake onthe walls of thehole.

As has been mentioned her'einabove, the most important physicalcharacteristic of a drilling mud is its ability to form a thinimpervious cake on the walls of the hole thereby sealing formationsagainst infiltration of water. The terms cake-forming and water-lossproperties, sealing properties and, as will be brought out hereinbelow,filtration characteristics or filter rate are used synonymouslythroughout this specification to denote this characteristic.

A' measure of the tendency for a mud to lose water to the formationsbeing drilled and to form a thin impervious cake upon the wall of thehole can be obtained by means of a simple filtration test. In fact, thescaling properties of the mud are almost entirely dependent upon thecharacter of the mud-cake formed when the latter is pressed against amembrane or filter permeable to Water and are very largely independentof the character of the membrane or filter employed. As a consequence,the filtration rate of a mud becomes of prime importance in determiningthe quality of the mud.

The procedure for determining filtration rate isdescribed in RecommendedPractice on Standard Field Procedure for Testing Drilling Fluids, A. P.I. Code No. 29, second edition, published July 1942, page 11, andconsists in measuring the total volume of filtrate water obtained duringa given time interval of pressure filtration. The amount of filtrateobtained in the first five minutes of filtration can be used as anindication of the relative quality of various muds. A more accurateevaluation is obtained from the volume of filtrate collected in thefirst fifteen minutes of filtration. An even better procedure is todetermine the total volume of filtrate in the first hour of filtration.Numerous correla tions between such tests and actual drilling experiencewith the same muds have shown that the muds yielding a total filtrate ofless than 30 ml. in the first hours filtration period are usually verysatisfactory. On the other hand, muds yielding a total filtrate inexcess of 45 ml.

tions at moderate depth, the permissible maximum filtration rate of themud may be as high as about 55 ml. of total filtrate in the first hourof filtration. A mud which yields a total filtrateof '30ml'-in;the-first-;hours filtration period will yield tabout 7 ml.*of-filtrate in the first fiveminutes and about 1'5 rnl.- of filtrate,in the first fifteen minutes of filtration. vsimilarly a mud whichyields a filtrate of 45 ml. in the first hour of filtration will giveapproximately 11 ml. in the-first five minutes and about 22 mLinthefirst fifteen minutes of filtration. A mud which is satisfactory fordrilling operations will under the conditions ofthis test, deposit afilter'cake of not over one-quarter 'inch,preferablyone-eighth inch, inthickness and of a soft, plastic or gelatinous texture, whereas mudsexhibiting unsatisfactory filter rates tend to deposit a thick, toughmud cake. Normally, as has already been mentionedhereinabove, mudspossessing acceptable "filtering characteristics form good mud'cakesand, therefore, in practice only the filtering rate is ordinarilyobserved. I 7

From the'foregoing, it may be concluded that in preparing ortreating adrilling mud in order to endow it with properties which will tend toinsure satisfactory performance in the field, it is desirable that thetreated mud when tested, as above described, yield a total filtrate ofnot over 55 ml. in the first hour-of filtration, and pref erably lessthan 30 ml. of filtrate, and that the deposited cake be preferably lessthan one-eighth inch in thickness and of a-soft gelatinous texture.

It has also been observed that if the water losses from the drillingfluid to the formations being drilled contains a dissolved salt ormixture of salts, such as for example sodium chloride and calciumchloride, in appreciable quantity, thereis less tendency for swelling ofthe formation-to occur. =In the production zone thisis particularlyimportant and is reflected in anincreased rate of production from theWell. This is particularly true ofthe polyvalent metal inorganic salts.Accordingly, in many cases it is desirable to incorporate a salt, suchas for examplesodium chloride, in drilling mud to .theextent of 1'% toor as much as 10% by weight of the total mud, or even more. Thepolyvalent metal salts are also effective in smaller amounts down toabout 0.1%. However, the addition of such quantitiesof a salt usuallyhave anadverse effect upon the water loss characteristics of the mud andit is necessary to control the performance characteristics of asalt-base mud by the addition of'treating agents.

As has already been mentioned hereinabove, a mud in order to be usable,must be capable of being readily and easily circulated by means of apump such as is ordinarily employed for the purpose in the field. From apractical standpoint, it has been found that within certain limits themore readily the mud can be circulated, the faster will drillingproceed. With many muds it has been observed that if their Marshfunnelviscosities, as determined by the 500 ml. in and 500 ml. out method, isin excess of 55 seconds they may exhibit impaired circulation rates. Onthe other hand, it has now been observed that certain materials whenadded to control water loss may result in muds having viscosities inexcess of. 55 seconds but which are readily pumpable in actual practice.

Insofar as the drilling operation and the subsequent production rate areconcerned the most important characteristic of a drilling mud is itsfiltering rate and the viscosity of the mud need only be such that it ispumpable. Throughout this specification and the claims, wherever theterms viscosity, Marsh viscosity, apparent viscosity or funnel viscosityare employed, they relate to the viscosity as determined by means of theabove test, a description of which can be found in Recommended Practiceon Standard Field Procedure for Testing Drilling Fluids,- A. l. 1. CodeNo. 29, second edition, published July 1912, page- 6, except thatsoomterma isineas ured into the funnel and the time determined for the500 ml. to run out.

It is therefore, one of the objects of this invention to provide for atreatment of a drilling mud which will cause the latter to have afiltration rate of less than approximately 45 ml. and atr'nost not morethan 55 ml. in the first hour of filtration, said treated mud having aviscosity sufficiently low so that it is pumpable.

When the viscosity of the mud is sufiiciently low to permit it to bereadily circulated and the filtration rate is within the limitsdescribed hereinabove, the mud will ordinarily possess sufiicientthixotropic properties for all practical purposes.

It is another object of this invention to provide for a treatment of adrilling mud containing an added salt resulting from the reaction of analkali metal or an alkaline earth metal witha strong mineral acid, suchas for example sodium chloride, potassium sulfate and calcium nitrate,said treatment causing the salt-base mud to have afiltration rate ofless than approximately 45ml. and at'most not more than 55 ml. in thefirst hour 'of filtration, said treated mud 'being pumpable.

The term performance characteristics is herein usesto include wallbuilding, water-loss and thixotropic prope'rties and viscosity but doesnot refer to the specific gravity of the mud.

' Inthe usual field operations, it is often necessary to form a cementplug in the hole and to subsequently'drill through this plug, therebycontaminating the mud with cement. Mud which has been contaminated withcement is termed cement-cut mud. It has been observed that suchmudsusually possess poor performance characteristics-in accordance withthe quality definitions given hereinabo-ve and that the greater thecontamination the poorer the quality. Cement-cut muds often become i-sov'iscc us'in character that it is difficult, and often impossible, tocirculate the contaminatedmaterial. This increase in apparent viscositymay impart gas cutting tendencies to the mods, that is, prevent theescape of gas from the mud, and tend 'to prevent the proper'releaseofcuttings therefrom. Further, as Wlll. be discussed more fully later,such muds form thick, pervious cakes upon the wall of the hole whichpermit the ready penetration of water into the formation.

Clays, which consist predominately of hydrated sili- 'cates of alumina,when suspended in water possess an appreciable electric charge,suchcharge ingeneral being of a negative character. On the other hand,cement as is well known comprises a complex mixture of compounds ofcalcium, magnesium, iron, aluminum and silicon. Although it has not beenestablished beyond question and it is not desired to be bound 'by thetheory, it is believed that the poor. performance characteristicsimparted by the presence of cement to an otherwisesatisfactory drillingmud is due in part to thecoagulation or flocculation of thenegativelycharged clay particles by the polyvalent positively chargedmetallic ions leached from the contaminating cement. Normally by far thelargest proportion of the positive ions leached from cement are calciumions. Muds may also become similarly contaminated during drillingoperations by coming in contact with penetrated formationswhich containmaterials, such as gypsum, from which polyvalent metallic ions may beleachedby the aqueous phase of the drilling fluid.

Furthermore, depending upon 'the' particular source, even the virginclays employed for preparing drilling muds may contain materials capableof yielding poiyvalent metallic ions when the clay is dispersed inwater. in any event, by whatever means the contamination maytake place,it appears thatthe presence'of'polyvalent metallic ions either insolution in the aqueous phase and/or in combination with the negativelycharged clay particles, is highly undesirable in tha't such a-conditionadversely affects the performance characteristic of the mud.

In the .past when drilling muds have become contam inated or, in anyevent, when their apparentviscosities have become undesirably high, ithas frequently been the practice to remove the mud from the hole and todispose of it as useless material. Such practice involved a considerabledisposal problem and, furthermore, it entailed considerable expense forthe purchase of new mud. It has been the practice in some fields tolower the viscosity of cement-cut muds by the addition of water. In thepast this has been highly undesirable inasmuch as such muds normallyexhibited excessively high filter rates.

Muds reclaimed 'by means of chemical treatment, unless properlycontrolled, will not be of high quality nor will they alleviate thedifficulties encountered from the caving of formations. The reason forthis is that a treatment which merely controls the viscosity of the mudis insufficient unless attention has also been given to the cake-formingand water-loss properties of the mud and their importance.

It has now been found that drilling muds can be treated with certainreagents which will control both the viscosity and the filtration rateof the mud and that it is possible to add the reagents to the mud eitherafter the contamination has taken placeor, in those cases where it isknown or expected that the mud is going to be contaminated byundesirable materials, such as for example, when it is anticipated thata cement plug will be drilled through and thatthe mud will then becomecontaminated with cement, the reagents can be added to the mud prior to'said contamination. This latter'type of treatment immunizes the mudagainst any substantial deterioration in its performance characteristicsupon subsequent admixing with the contaminating material, and in somecases it has been found that such contamination afterthe' addition ofthe reagents which have now been discovered even improves theperformance chjaracQ teristics of the mud. Furthermore, thesereagents=are so effective in controlling water loss that the viscosityif desired can be controlled merely by the addition of water to the mudeither before or after contamination with the cement or similarmaterials. Illustrative of this point it has been observed that'when mudbecomes contaminated with cement its viscosity and filtration ratebecomes undesirably high, but that these factors can be reducedtodesirable values by the addition to the contaminated, mud of a mixtureof treating agents as described below. On the other hand, by adding thismixture to the mud prior to contamination with cement, the performancecharacteristics of the mud are improved, and

upon the subsequent addition of cement the performance characteristicsare still acceptable and in many. cases may remain substantiallyunchanged or may even be improved.

It is another object of. this invention to provide for a treatment ofmud which willsimultaneously control cake-forming and water lossproperties, the viscosity and the thixotropic properties of the mud andwhich will not markedlyalter the specific gravity of the mud. H

It is an additional object of this invention to provide ifor acombinationv treatment of muds comprising the addition of one or morematerials which will control the viscosity of the mud without acceptablyaltering its cakeforming properties and a second material or combinationof materials which will control the cake-forming properties of the mudwithout acceptably controlling. its

viscosity.

It is also an object of, this invention to provide for. r thepretreatment of mud in order to render it immune to the effects ofsubsequent contamination with cement or similar materials therebyproviding for retaining the mud at all times in excellent condition interms of the desirable properties enumerated hereinabove.

It has been discovered that by a suitable choice of treatment, not onlycan the viscosity of a mud,.and par:

ticularly of a contaminated mud, be controlled, but the cake-forming andwater-loss properties of the mud, as measured by the filtration rate,can also simultaneously be regulated and maintained at a high quality.It has further been determined that controlling the viscosity of a mud,and especially of a cement-cut. mud, does not necessarily control thefiltration rate of the mud.

It is desirable in practicing this invention to improve the performancecharacteristics of a mud not to employ an amount of treatingagent oragents in excess of the minimum amount necessary to obtain the desiredperformance characteristics. If the quantity of reagent exceeds thisminimum amount in any great excess the mud may be deleteriouslyaffected. Normally the reagents are added in relatively smallproportions in the order of about 0.01% to 2.0% by weight based upon theWeight of drilling fluid treated, although under some circumstances asmuch as 5% by weight or even 10% by weight of some of the reagents maybe used. By the application of the above described tests it may readilybe determined what the necessary amount of any given reagent or reagentsis for any mud.

Before considering the types of reagents comprising the subject matterof this invention, it should firstbe emphasized that, as might beexpected, naturally occurring clays and the muds prepared from them vary,considerably in character. For example, they differ in ultimatechemical composition, in amounts and types of colloidal material, and inamounts and types of impurities. Furthermore, the common contaminantswhich may become included in the mud during its use in drillingoperations, namely, calcium hydroxide leached from cement, and gypsum,differ in type, one being a fairly strong base and the other a neutralsalt. As a consequence and in view of the complex character of colloidaldispersions, it isronly logical to expect that in "general theperformance characteristics of muds prepared from clays of differentorigins or even of the same mud contaminated with different materials,will not necessarily be affected in ex actly the same manner by theaddition of any given treating agent. In spite of these variations ithas been found that the treating agents disclosed in the specificationwhen added in proper amount will control the performance characteristicsof various muds within acceptable limits The methods employed for makingperformance tests have been outlined in detail above. In determining theeffect of 'a treating agent or agents on a mud, the pro cedure employedin the laboratory has been to add the desired amount of treating agentor agents to the mud followed by a thorough agitation of the mixture forone hour prior to the conducting of the performance tests. It will beobserved that such a procedure completely eliminates any necessity formaking a chemical analysis of the mud and, as a consequence, it has beenfound to be the most practical method which can be employed in thefield.

It has been discovered that the performance characteristics of a mud andparticularly its water loss properties can be controlled by the additionto the mud of quebracho as one treating agent, along with a salt of anoil soluble mineral oil sulfonic acid as a second treating agent. Thelatter include salts of the alkali metals such as lithium, potassium andsodium, the ammonium salts, and salts of the various amines andsubstituted amines such as for example dipropyl amine andtriethanolamine; and the higher molecular weight oil soluble sulfonicacids either singly or in admixture with one another. The sulfonic'acids of relatively high molecular Weight employed to organic acidswhich pass into the sludge. After separation of the sludge the mahoganyacids which are preferr'ed may be recovered in the form of sodium saltsby treatment of the acid-treated oil with sodium hydroxide to producethe sodium sulfonates which are then subsequently removed from the oilsolution by extraction with alcohol. The other alkali metal salts andthe ammonium salts may be obtained from the above sodium salts by wellknown processes of metathesis. An example of a commercially availableoil-soluble alkali metal salt of a higher molecular weight sufonic acidis a concentrate in lubricating oil, comprising about 60% sodiumsulfonates and 40% lubricating oil.

Desirably, but not necessarily, the salts of oil-soluble sulfonic acids,may be dispersed in a petroleum or other hydrocarbon oil such as sprayoil, a transformer oil extract produced by Edeleanu extraction ofa-suitable petroleum distillate with sulfur dioxide, a light lubricatingoil or even a heavy lubricating oil, prior to introduction into thedrilling fluid. Oils of intermediate degree of volatility, such as sprayoil, kerosene or gas oil, or selective solvent extracts from these, havean additional beneficial effect of reducing the foaming tendency of themud. The amount used is preferably between about :4:and about 7% byweight of the finished mud although llarger amounts up to about 25% maybeused in many muds-to advantage.

It has further been discovered that the alkali metal-and ammonium acidand neutral salts of the various 'acids of phosphorus such asortho-phosphoric acid, pyrophosphoric acid, hexametaphosphoric acid,etc., when added to drilling fluids in conjunction with thequebrachoandsulfonate salts disclosed hereinabove or only withquebra'cho,-particularly when the drilling fluid is contaminated-withcement or similar materials yielding polyvalentfmetal ions, results inperformance characteristics which are ib'etter than can be obtained bythe use of these materials' alone.

One factor which has not been previously mentioned concerning thechemical treatment of muds'isfbasedon the fact that in actual commercialuse muds may be exposed to temperatures of as high as 150 F. to'250" F.,the temperature depending principally upon the location and the depth atwhichthe drilling is being "conducted. It has been observed thatmanymuds which have been chemically treated-to improve their performance"characteristics tend to deteriorate'with respect to'thesecharacteristics upouprolonged exposure to temperatures in theneighborhood of 200 F. to 250 R, or even lower as is the case indrilling operations. This is apparently due to the fact that thechemicals added may be hydrolyzed or otherwise affected in such a manneras toalter their original effect upon the performance characteristics ofthe mud. It might be thought,'therefore, that the use of chemicalsreacting in this way would be a bar to their practical'application inthe treatment of muds, and. in those cases where the degradation withrise in temperature is rapid, such as in the case of sodiumhexametaphosphate, their use may be inadvisable. However, it has beenfound that many chemicals, which apparently only slowly lose theireffectiveness upon exposure to heat, may be utilized. In such cases itis preferable to add the chemical to the mud each day, therebymaintaining a low filtration rate and a low viscosity. The addition ofthree times as much chemical as is required to maintain thedesiredperformance characteristics over a period of one day will notmaintainthe performance characteristics over a period of three days. Ofcourse, it is' evident that wherever it is feasible, it is preferable touse chemicals whose effect up'on'theperformance characteristics of a mudare not altered by prolonged exposure to-elevated temperatures such asmay be encountered in the particular zone being drilled, and if suchchemical will give the desired performance characteristics according tothe tests herein disclosed, such'a'chemical is the most desirable. Insorne-'cases,"-it has been observed that a' certainorder of additionwill give better performance characteristics thanthereverse order ofaddition. In the examples given below, therefore, unless otherwisenoted, it will be considered that the chemicals-are added in the orderin which they are listed, since they give, when added in such order, themost desirable results. i v

Ithas further been discovered that the performance characteristics of asalt base mud, and particularly its waterless properties can becontrolled by the addition to the mud of quebracho and a salt of oilsoluble sulfonic acids.

In the practice of this invention in the field, the treating agents .mayconveniently be added to the circulating mudstream at a point adjacentto the mud pump suction inlet in the mud sump. Thorough admixture of thethus introduced reagents may be assured by rapid recirculation of themud from the mud sump through .a .spare slush pump. .During treatment,mud samples may be taken from the circulating mud stream at frequentintervals and tested in order to determine when the desired degree oftreatment has been'effected.

The following examples are presented in the nature of illustrations ofthe practical value of the processes of the invention 'and are not to beconstrued as limiting the invention in any sense.

. Example I A Santa Maria Valley clay was mixed with water .to give afluid weighing 78.5 pounds per cubic fOO'L' To a given volume of thisfluid was added an equal volume of water in which had been dispersedvarious treating agents and the viscosity and filtration rate weredetermined on each sample by the procedures already described. Thedata-from these several experiments are given in the followingtabulation:

able concentrate comprising about 60% by weight of the sodium salts ofmahogany or oil soluble sulfonic acids prepared by sulfuric acidtreatment of lubricating oil, and about 40% of lubricating oil.

Example 11 I The samefluid was used as for the'experiments described inExamplel. To a given volume of this fluid was added an equal volume ofwater in which had been dispersed various treating agents, andsubsequently the drilling fluid was admixed with 0.7% by weight ofhydrated cement. The viscosities and filtration rates of these sampleswere determined and are recorded in' the following tabulation:

. Filtration Rate, ml. Treating Agent Added, Percent Marsh Visby Weightof the Final-Drilling cosity,-500/ Fluid 500, Secs. 1st 5 1st 15 Min.Min.

a. No added treating agent 22 25.0 14.0 b. 2.0% sodium sulfonate 22 10.0

c. 2.0% sodium sultonate, 0 5% qu bracho 21 1. 7 4 2 d. .5% quebracho.35 20.0 35 0 Example'I shows that in a typical water base ,mud .ofrelatively high filtration rate, the addition of sodium sulfonate aloneimproved the filtration rate appreciably without affecting the viscosityappreciably, and the addition of quebracho alone had little efiectonfiltration ratewith an adverse effect on viscosity; whereas with thecombination of quebracho andsulfonate the filtrationrate ;was

, 9 exceptionally low and the adverse effect of the quebracho onviscosity was substantially completely overcome.

Example II shows that in the presence of cement, the effects are quitesimilar. The mud is immunized against cement contamination. It may beobserved that in both examples the eifect of the quebracho on thefiltration rate in the absence of the sulfonate was a reduction of onlyabout 19 to 20 percent, whereas in the presence of sulfonate, thereduction Was 55 to 58 percent.

While the above constitute specific examples illustrating the invention,it is apparent from the foregoing that the invention is not limitedthereto. By substituting the other sulfonate salts described above, orby employing the oils, salts of acids of phosphorous, and/ or otheradditives described above, similar beneficial effects of the combinationof quebracho and sulfonate are obtained. Re- 'sults in salt base mudsare similar to those observed above in Example II for the cement cutmuds.

This application is a continuation-in-part of our copending applicationSerial Number 117,934, filed September 26, 1949, now issued as U. S.Patent No. 2,655,475, which was in turn a continuation-in-part of ourcopending application Serial Number 686,726, filed July 27, 1946, andnow issued as U. S. Patent Number 2,518,439.

Other modifications of this invention which would occur to one skilledin the art may be made, and these are to be considered within the scopeof the invention as defined in the following claims.

We claim:

1. A drillingfluid comprising water, a finely divided solid suspendedtherein, between about 0.01% and about 10% by weight of a salt of an oilsoluble mineral oil sulfonic acid, and between about 0.01% and about 10%by weight of quebracho.

2. A drilling fluid according to claim 1 containing about 1.2% sodiumsulfonate and about 0.5% of quebracho.

3. A drilling fluid according to claim 1 which also contains betweenabout 0.4% and about 25% by weight of a hydrocarbon oil.

4. A drilling fluid according to claim 1 which also contains betweenabout 0.01% and 10% by weight of an alkali metal salt of an acid ofphosphorus.

5. A drilling fluid stable to salines encountered in drilling, whichcomprises water, clay, quebracho, and a salt of an oil soluble mineraloil sulfonic acid, said quebracho and said salt being present in amountsbetween about 0.01% and about 10% by weight, not greatly in excess ofthe minimum amounts necessary to cause the drilling fluid to have afiltration rate less than approximately ml. in the first hour offiltration and a viscosity suificiently low to be pumpable.

6. A drilling fluid according to claim 5 in which the salt is an alkalimetal salt.

7. A drilling fluid according to claim 6 in which the alkali metal issodium, and the quebracho and the salt are each present in amountsbetween about 0.01% and about 2.0% by weight.

References Cited in the file of this patent UNITED STATES PATENTS2,331,049 Schindler Oct. 5, 1943 2,343,113 Jones Feb. 29, 1944 2,491,437Perkins Dec. 13, 1949 2,550,236 Fischer et al. Apr. 24, 1951 2,568,992Doscher Sept. 25, 1951 OTHER REFERENCES Rogers: Composition andProperties of Oil Well Drilling Fluids, 1st edition, page 306, pub. 1948by Gulf Pub. Co. of Houston, Texas.

1. A DRILLING FLUID COMPRISING WATER, A FINELY DIVIDED SOLID SUSPENDED THEREIN, BETWEEN ABOUT 0.01% AND ABOUT 10% BY WEIGHT OF A SALT OF AN OIL SOLUBLE MINERAL OIL SULFONIC ACID, AND BETWEEN ABOUT 0.01% AND ABOUT 10% BY WEIGHT OF QUEBRACHO. 